This invention relates to in situ measurements in a cased well borehole which can determine at least qualitatively the entry of fluid into the cased borehole and trace its movement through the cased borehole.
More particularly, the invention relates to a pulsed neutron well logging technique in which simultaneous determinations are made of the thermal neutron lifetime or thermal neutron decay time in a cased borehole and the movement of borehole fluid is traced through oxygen activation measurements and fluid sample measurements also made simultaneously in the well borehole.
Recent innovations in the art of pulsed neutron well logging by the assignees of the present invention have included the introduction of simultaneous measurements of earth formation and borehole thermal neutron decay time or thermal neutron life times in a dual detector small diameter (1-11/16 inch) instrument which is sized and adapted to be passed through production tubing into a cased well borehole. The thermal neutron capture cross sections of the borehole and earth formation surrounding the borehole are determined. The instrumentation can provide a variety of other information. For example, the radiation of the borehole and its environs with 10 MEV pulsed neutrons can result in the transmutation of oxygen 16 into nitrogen 16 plus a proton. This isotope of nitrogen is unstable and beta decays back into oxygen with a 7.35 second half life. The excited state of oxygen 16 thus produced immediately emits a 6.13 MEV gamma ray which can be detected by the gamma ray detectors on the dual detector pulsed neutron logging instrument as well as by any additional gamma ray detectors carried along the tool string with the dual detector instrument.
The operation and configuration of the dual detector pulsed neutron logging instrument referred to in the foregoing description are detailed with particularity in U.S. Pat. Nos. 4,350,888, 4,409,481, and 4,388,529 which are assigned to the Assignee of this application and are incorporated herein by reference. Details of the operation of this well logging system, hereafter referred to as the TMD system (Thermal Multigate Decay) will not be gone into in detail in the instant application. However, the portion of the operation of the instrument which is particularly germane to the instant disclosure will be discussed in more detail subsequently.
If the relative movement of the dual detector pulsed neutron logging instrument such as that used in the TMD system previously mentioned (and hereinafter referred to as the TMD well logging instrument) and the activated oxygen in well fluid moving through a well borehole meets the criteria laid out in more detail subsequently herein, it may be possible for the dual detector instrument to detect the flow of well fluid within the cased well borehole. The fluid moves past the detectors of the dual detector instrument and also past any additional gamma ray detectors which might be included in the tool string. Such oxygen activation techniques can be used for determining fluid salinity and are disclosed in U.S. Pat. No. 4,574,193 which is assigned to the current assignee and incorporated herein by reference.
Observations made in the time gates used for detecting background radiation in the pulsed neutron TMD well logging instrument can be used to detect oxygen activation in fluid in a well borehole which moves with respect the well logging instrument in a particular direction. Moreover, by moving the TMD well logging instrument through the well borehole at different speeds, the flow rate of the fluid may be determined if it lies within particular ranges at which the instrumentation would have optimum sensitivity. Because of the difference in the carbon to oxygen ratio of hydrocarbons and water, it may be possible to determine, qualitatively at least, whether the fluid moving past the detectors comprises a substantial water cut.
In the regard of determining whether the fluid moving past the detectors and oxygen activated comprises a fluid having a particular water cut, a fluid sampler is provided in the tool string in the present invention with the TMD well logging instrument along with a third gamma ray detector and casing collar locator. The fluid sampler may be used for capturing or taking fluid samples of the fluid at a particular location in a well bore in order to resolve ambiguous situations which can arise through the use of the TMD well logging instrument as an oxygen activation well logging instrument.